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Abstract

Liquid-crystal spatial light modulators (LCM) are a common tool to tailor femtosecond laser pulses. The phase stability of 1 kHz, sub-20 fs visible shaped and unshaped pulses are investigated. Our results show that the spectral phase after the LCM varies from pulse to pulse leading to strong deviations from the predicted pulse shapes. This phase instability is generated only by LCM and is strongly temperature dependent. Based on the experimental data, a numerical model for the phase was developed that takes the temperature-dependent phase instability as well as pixel coupling across the LCM into account. Phase stability after the LCM can be improved by an order of magnitude by combining the control the temperature of the LCM and by using rapid-scan averaging. Reliable pulse shapes on a pulse-to-pulse basis are crucial, especially in coherent control experiments, where small differences between pulse shape are important.

Power spectra of the experimental SH signal on a linear (a) and log-log (b) scale without 4f-setup (black squares), after the non-cooled LCM 1 (red circles) and after the cooled LCM 1 (blue triangles).

A comparison between (a)-(c) step-scan and (d)-(f) rapid-scan averaging. For both schemes 70 data points are calculated from the SH signal of 4900 adjacent laser pulses; (a), (d) without 4f-setup (black squares), with (b), (e) non-cooled (red dots) and (c), (f) cooled (blue triangles). The gray dashed line is a guide to the eye for the deviation of the points from the mean over the signals from all 4900 laser pulses. (g) Average scheme using step scan and rapid scan for the same set of data.

Simulation of shot-to-shot SHG-AC traces for the non-cooled, TL cases for pulses with pulse durations of (a) 30 fs and (b) 100 fs. Particular traces are selected and highlighted in color. Phase noise was the same as in Fig. 4.